Count appearances of a string in another - JavaScript

We are required to write a JavaScript function that takes in two strings and returns the count of the number of times the first string appears in the second string.

Let's say our string is:

const main = 'This is the is main is string';

We have to find the appearance of the below string in the above "main" string:

const sub = 'is';

Using Regular Expression with replace()

Let's write the code for this function using a regular expression approach:

const main = 'This is the is main is string';
const sub = 'is';

const countAppearances = (main, sub) => {
    const regex = new RegExp(sub, "g");
    let count = 0;
    main.replace(regex, (a, b) => {
        count++;
    });
    return count;
};

console.log(countAppearances(main, sub));

4

Using match() Method

A simpler approach uses the match() method which returns an array of matches:

const main = 'This is the is main is string';
const sub = 'is';

const countAppearances = (main, sub) => {
    const regex = new RegExp(sub, "g");
    const matches = main.match(regex);
    return matches ? matches.length : 0;
};

console.log(countAppearances(main, sub));

4

Using split() Method

Another approach splits the string by the substring and counts the parts:

const main = 'This is the is main is string';
const sub = 'is';

const countAppearances = (main, sub) => {
    return main.split(sub).length - 1;
};

console.log(countAppearances(main, sub));

4

Handling Edge Cases

Here's a more robust version that handles empty strings and case sensitivity:

const countAppearances = (main, sub, caseSensitive = true) => {
    if (!main || !sub) return 0;
    
    const flags = caseSensitive ? 'g' : 'gi';
    const regex = new RegExp(sub.replace(/[.*+?^${}()|[\]\]/g, '\$&'), flags);
    const matches = main.match(regex);
    return matches ? matches.length : 0;
};

// Test cases
console.log(countAppearances('This is the is main is string', 'is')); // 4
console.log(countAppearances('Hello World', 'world', false)); // 1 (case insensitive)
console.log(countAppearances('', 'test')); // 0
console.log(countAppearances('test', '')); // 0

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1
0
0

Comparison

Conclusion

The split() method provides the simplest and most efficient solution for counting substring occurrences. Use the match() method when you need regex features like case-insensitive matching.